The present invention relates to a method for the detection of rotational speed of a wheel or any other rotating body by means of a sensor, which is coupled by way of a magnetic field to an encoder (5, 13, 23) that rotates with the rotating body. The signals of at least two separated signal paths (S1, f1; S2, f2) having at least one own sensor element (S, S1, S2) are compared with one another, and depending on the result of comparison, one of the two signal paths is connected to a subsequent electronic control unit (ECU) (25). The present invention also relates to an arrangement for detecting the rotational speed of a wheel or any other rotating body by means of a sensor (24), which is coupled by way of a magnetic field (H1, H2) to an encoder (23) that rotates with the rotating body. The sensor (24) includes two separated signal paths (S1, f1; S2, f2) having at least one own sensor element (S, S1, S2) and preferably an own signal conditioning stage (26, 27), the output signals of which are compared in a comparator (28), and in that depending on the result of comparison of the comparator (28), the first signal path (S1, f1) or the second signal path (S2, f2) of the two signal paths is connectable to a subsequent electronic control unit (ECU) (25). The invention also relates to a sensor module for avoiding or suppressing the measurement of interfering magnetic field components, which are modulated by movements of a magnet encoder (5, 13, 23, 37), composed of at least one first (35, 45, 50, 54) and one second magnetic field sensor element (34, 46, 52, 55) and at least one bias magnet (36, 39). With regard to a system of Cartesian coordinates, the magnet encoder (5, 13, 23, 37) with respect to its coding surface defined by its radii is arranged substantially in parallel to the x-y plane, in that the bias magnet (36, 39) with respect to its direction of magnetization and the magnetic field sensor elements (34, 35, 45, 46, 50, 52, 54, 55) with respect to their respective sensor surface are aligned essentially in parallel to the coding surface of the magnet encoder (5, 13, 23, 37) and, thus, in parallel to the x axis, and in that the first magnetic field sensor element (35, 45, 50, 54) and the second magnetic field sensor element (34, 46, 52, 55) are arranged differently as regards their distance from the magnet encoder (5, 13, 23, 37) in the z direction.
Active sensors for the automotive industry are known in the art in several cases. They exist in two-wire designs and three-wire designs. The description of the invention is based on the example of the two-wire design, as is conventional practice in brake systems. It is, however, covered by the invention to employ the basic idea also with respect to three-wire designs, being customary e.g. in motor applications and/or gear applications.
This state of the art is represented in FIG. 1. In FIG. 1a, a sensor 1 and an ECU (Electronic Control Unit of the ABS controller, or generally an electronic check unit) 2 are electrically connected to each other by way of a two-wire line 3, 4. The operation of the sensor requires an operating voltage VB which is provided by the ECU at terminals K1, K2. A signal current IS flows back to the ECU via the sensor, its strength varying in the beat of a rotational speed information, which is generated by an encoder 5 and decoded in the ECU. In FIG. 1b, a sensor 6 and an ECU 7 are interconnected electrically by way of a three-wire line 8, 9, 10. The operation of the sensor requires an operating voltage VB in this case, too, which is provided by the ECU at terminals K1′, K2′. The sensor returns a signal current VS containing the sensorial information to the ECU via the terminal K3.
FIG. 2 shows the inside system configuration of two typical variants of active wheel rotational speed sensors with a two-wire interface. Sensors for the unidirectional rotational speed detection without additional functionalities can be schematized according to FIG. 2a. The wheel rotational speed sensor 1 comprises a sensor module composed of the magnetoresistive sensor element S, which is connected to an electronic signal conditioning stage SC. The sensor element is coupled to the encoder E by way of a magnetic field H. The encoder rotating at wheel rotational speed modulates the air slot field H with an incremental pattern, which contains the wheel rotational speed information. Sensor element S and the signal conditioning stage SC produce from this air slot field modulation a signal voltage for controlling a modulator stage M, which in turn controls a current source 11 so that the incremental pattern of the encoder is represented as a load-independent signal current Is1. Sensors for the bidirectional detection of wheel rotational speeds and/or for the transfer of additional (diagnosis) parameters can be schematized according to FIG. 2b. In contrast to before, the signal conditioning stage is divided into the paths WS and ZI. Stage WS is used to condition the wheel rotational speed information from the encoder signal, while ZI serves for the separate conditioning of additional information from the sensor/encoder interface. Such additional information can e.g. be the direction of rotation and the air slot size. In a signal stage SL, the signals conditioned by WS and Z1 are joined to a control signal for the modulator stage M, which in turn controls a current source 11 so that the protocol of wheel rotational speed functions and additional functions contained in the control signal is imaged as a load-independent signal current Is2. According to the state of the art, a three-level protocol or a PWM protocol (pulse width modulation) is employed at present.
It is possible to use ferromagnetic toothed wheels or perforated discs, on the one hand, which produce a variable magnetic air slot in combination with a permanent magnet. On the other hand, permanently magnetized north-south pole areas can be used, which are embedded in alternating sequence into a wheel bearing seal, for example. The necessary auxiliary magnets are integrated in the sensor as a mechanical component of the sensor module. The explanation of the invention hereinbelow is limited to the basic technical application, i.e. the combination of magnetoresistive sensors with permanently magnetized encoders, however, it is possible to the expert in the art to apply the principles of the invention directly to combinations with ferromagnetic encoders, what is also in the sense of the invention.
FIG. 3 shows directions of reference and characteristic curves herein used for explaining specific physical circumstances of magnetoresistive sensor elements in connection with the improvements aimed at by the invention. FIG. 3a shows a magnetoresistive special sensor module 12 according to FIG. 2a in its geometric alignment to a permanently magnetized encoder track 13 during wheel rotational speed operation. The encoder track lies flatly in the XY plane and moves relative to the sensor element in the Y direction. Part 14 of the sensor module comprises a bridge circuit 16 made up of four magnetoresistive perm alloy (barber pole) resistors 18, as illustrated in FIG. 3b. The plane of the resistance layers extends like the plane of the encoder in parallel to the XY plane. FIG. 3c shows the operating characteristic curve of signal voltage Vss as a function of the magnetic air slot field strengths Hy, Hx1 and Hy, Hx2. Herein, Hy is the magnetic field component in the moving sense of the encoder track, and Hx is a magnetic field component of the encoder in the transverse direction hereto. Hx1 and Hx2 are field components in x direction being oriented opposite each other. FIG. 3c shows that alternating HX1, HX2 components of the encoder magnetization lead to mirror-inverted characteristic curves 18, 19. The alternation of the characteristic curve linked to the alternating signs of the Hx components occurs abruptly and is referred to as ‘flipping’. Flipping leads to unwanted corruption (doubling) of the encoder signal and is disturbing for the rotational wheel speed detection. In the current practice, flipping is avoided because a so-called bias magnet 15 with a polarization in the X direction produces a so-called supporting field, which is larger than the components Hx1, Hx2 and therefore fixes one of the two characteristic fields, e.g. 18.
FIG. 4 serves to explain the causes for the appearance of magnetic components of the encoder in the X direction. FIG. 4a shows an encoder, as viewed from the XZ plane, with its magnetic field 20, exiting from the magnetic track 21 to a ferromagnetic return path made of sheet metal 22. The lines of electric flux of the encoder track exit in a broad middle zone in parallel in the Z direction. In the marginal zones the direction of exit additionally tends to the X direction. These Hx components have an inhomogeneous character and are not desirable as regards rotational speed measurement. With an only small offset of the sensor module 12 in relation to the center line, as is illustrated in FIG. 4a, no appreciable Hx component becomes active so that the field strength of the bias magnet 15 always prevails and the characteristic curve 18 is stably impressed.
With an encoder according to FIG. 4b, the sensor, with the same offset relative to the central position, will move into a range of lines of electric flux, which contains already major Hx components so that the stability of the characteristic curve is jeopardized. Jeopardy becomes critical when the encoder has only a narrow magnetic reading track and/or a strong magnetization, and/or the sensor module is positioned relatively far outside the middle of the reading track, and/or the sensor module is positioned very closely at the encoder surface. Therefore, the very strong influence of one of these parameters or the combination of several of these parameters can cause a critical case where flipping as referred to hereinabove will develop.
An object of the invention is to counteract the occurrence of flipping as described hereinabove, or prevent it, or in general to disclose a technology, which enhances the inherent safety of a sensor module in such a manner that unwanted flipping is prevented, or suppressed, or automatically detected, and the sensor module signals this condition to the ECU.